1. Field of the Invention
This invention relates to gallium nitride bulk crystals and methods for making the same.
2. Description of the Related Art
(Note: This application references a number of different publications as indicated throughout the specification by one or more reference numbers within brackets, e.g., [Ref. x]. A list of these different publications ordered according to these reference numbers can be found below in the section entitled “References.” Each of these publications is incorporated by reference herein.)
The usefulness of gallium nitride (GaN), and its ternary and quaternary alloys incorporating aluminum and indium (AlGaN, InGaN, AlInGaN), has been well established for fabrication of visible and ultraviolet optoelectronic devices and high-power electronic devices. These devices are typically grown epitaxially on heterogeneous substrates, such as sapphire and silicon carbide, since GaN wafers are very expensive. The heteroepitaxial growth of group III-nitride causes highly defected or even cracked films, which deteriorate the performance and reliability of these devices.
In order to eliminate the problems arising from the heteroepitaxial growth, GaN wafers sliced from bulk GaN crystals must be used. However, it is very difficult to grow a bulk crystal of GaN, since GaN has a high melting point and high nitrogen vapor pressure at high temperature.
Up to now, a few methods such as high-pressure high-temperature synthesis [Ref 1, 2] and a sodium flux method [Ref. 3, 4] have been used to obtain bulk group III-nitride crystals. However, the crystal shape obtained by these methods is a thin platelet because these methods are based on a Ga melt, in which nitrogen has very low solubility and a low diffusion coefficient.
A new technique is based on supercritical ammonia, which has high solubility of source materials, such as polycrystalline GaN or Ga metal, and which has high transport speed of dissolved precursors. This ammonothermal method [Ref 5-9] has a potential for growing large GaN crystals. However, existing technology is limited by the crystal size, because the growth rate is not fast enough to obtain large crystals.